Radiographic imaging such as x-ray imaging has been used for years in medical applications and for non-destructive testing.
Normally, an x-ray imaging system includes an x-ray source and an x-ray detector array consisting of multiple detectors comprising one or many detector elements (independent means of measuring x-ray intensity/fluence). The x-ray source emits x-rays, which pass through a subject or object to be imaged and are then registered by the detector array. Since some materials absorb a larger fraction of the x-rays than others, an image is formed of the subject or object.
An example of a commonly used x-ray imaging system is an x-ray Computed Tomography (CT) system, which may include an x-ray tube that produces a fan- or cone beam of x-rays and an opposing array of x-ray detectors measuring the fraction of x-rays that are transmitted through a patient or object. The x-ray tube and detector array are mounted in a gantry that rotates around the imaged object. An example illustration of a CT geometry is shown in FIG. 1.
The dimensions and segmentation of the detector array affect the imaging capabilities of the CT apparatus. A plurality of detector elements in the direction of the rotational axis of the gantry, i.e. the z-direction of FIG. 1 enables multi-slice image acquisition. A plurality of detector elements in the angular direction (ξ in FIG. 1) enables measurement of multiple projections in the same plane simultaneously and this is applied in fan/cone-beam CT. Most conventional detectors are so called flat-panel detectors, meaning that they have detector elements in the slice (z) and angular (ξ) directions.
X-ray detectors made from low-Z materials need to have a substantial thickness in the direction of the x-ray beam in order to have sufficient detection efficiency to be used in CT. This can be solved by, for example, using an “edge-on” geometry, as in U.S. Pat. No. 8,183,535, in which the detector array is built up of a multitude of detectors, which comprise thin wafers of a low-atomic number material, oriented with the edge towards the impinging x-rays. An example illustration of a CT geometry using edge-on detectors is shown in FIG. 2, showing the position of the source 60, the direction of the x-rays 45, the detector array 50, a single edge-on detector 5 and the angular direction of movement of the array 55. It is common that each detector has a plurality of detector elements on a 2D grid on the wafer.
FIG. 3 is a schematic diagram illustrating a semiconductor detector module implemented as a multi-chip module similar to an exemplary embodiment in U.S. Pat. No. 8,183,535. In this example, the detector elements are organized in three depth segments 15 with respect to the direction of the incoming x-rays 45. This example shows how the semiconductor sensor also can have the function of substrate 5 in a Multi-Chip Module (MCM). The signal is routed 37 from the detector elements 15 to inputs of parallel processing circuits (e.g. ASICs) 30. It should be understood that the term Application Specific Integrated Circuit (ASIC) is to be interpreted broadly as any general circuit used and configured for a specific application. The ASIC processes the electric charge generated from each x-ray and converts it to digital data, which can be used to obtain measurement data such as a photon count and/or estimated energy. The ASICs are configured for connection to digital data processing circuitry 20 so the digital data may be sent to further digital data processing and/or memories located outside of the MCM and finally the data will be the input for image processing to generate a reconstructed image.
For a given rotational position, each detector element measures the transmitted x-rays for a certain projection line. Such a measurement is called a projection measurement. The collection of projection measurements for many projection lines is called a sinogram. The sinogram data is utilized through image reconstruction to obtain an image of the interior of the imaged object. Each projection line (a point in the sinogram) is given by an angular coordinate, θ, and a radial coordinate, r, as defined in FIG. 4. Each measurement with a detector element at a specific coordinate given by (r, θ) is a sample of the sinogram. More samples in the sinogram generally lead to a better representation of the real sinogram and therefore also a more accurately reconstructed image. An example of how a detector array, similar to that displayed in FIG. 1, samples the sinogram space is shown in FIGS. 6A-B for two different angular positions of the gantry separated by Δθ. The different r positions of the samples come from the different detectors in the array.
Generally, the gantry rotates continuously and each detector element measures the x-rays flux within a frame time. A measurement period, T, is here defined as the interval in time during which a certain detector element is occupied with a measurement. The length of the measurement period can be, but does not have to be, equal to the frame time. The measurement period is much smaller than the total data acquisition time and multiple measurement periods follow directly after each other throughout the overall data acquisition/measurement. The length of the measurement period is referred to as the temporal sampling interval and the reciprocal of the sampling interval 1/T is referred to as the sampling frequency. The angular sampling interval of the CT system is given by the angular velocity of the gantry, ω=dθ/dt, and the temporal sampling interval, T, via Δθ=ωT. A schematic example illustration of the angular sampling is displayed in FIG. 5, where the detector and the source are illustrated for two different positions separated in time by the sampling interval T. The radial coordinate for all projection lines corresponding to a specific detector element is invariant to the rotation of the gantry.
In order to perform an accurate image reconstruction from tomographic data, it is essential that there is a sufficient amount of angular samples. Insufficient angular sampling can lead to artifacts in the image, aliasing and poor resolution.
One way to increase the angular sampling frequency is to decrease the temporal sampling interval T. Decreasing the temporal sampling interval results in a corresponding increase in the amount of produced data.
The temporal sampling rate can be limited by the capacity of the data transfer from the measurement circuit, rather than the measurement circuit itself.